This invention relates to the use of particulates formed by curing fly ash with phosphonate scale inhibitors in subterranean operations.
Oilfield fluids (e.g., oil, gas, and water) are generally complex mixtures of aliphatic hydrocarbons, aromatics, hetero-atomic molecules, anionic and cationic salts, acids, sands, silts, clays and a vast array of other components. The nature of these fluids combined with sometimes severe conditions of heat, pressure, and turbulence to which they are often subjected during retrieval, are contributory factors to scale formation in oil and/or gas production wells and surface equipment. Wherever water production occurs, the potential for some type of scale formation exists. Scale, as the term is used herein, refers to a mineral or solid salt deposit that forms when the saturation of formation water to one or more minerals is affected by changing physical conditions (such as temperature, pressure, or composition); thus causing minerals and salts previously in solution to precipitate into solids. Scale deposits can form on any surface in a down hole operation, including subterranean formations, production tubing, gravel packing screens, and other well bore equipment. Scale can develop almost immediately, or build up over several months before becoming noticeable. The effect scale has on productivity depends on the type, location, and the mass deposited. Scale formation can become so severe as to restrict or even completely choke production. The formation of scale can decrease permeability of the subterranean formation, reduce well productivity and shorten the lifetime of production equipment. In order to clean scale from wells and equipment it is generally necessary to stop production, which is both time-consuming and costly.
The formation of scale is often controlled by the use of scale inhibitors. Several methods are known in the art for introducing scale inhibitors into production wells. For instance, a solid form of a scale inhibitor may be placed into the formation; however, this method is limited due to the fact that there are relatively few effective solid scale inhibitors and each has functional or design limitations. Another known way to place scale inhibitor is to impregnate a porous particulate with a scale inhibitor wherein the pores are in communicate with the fluid surrounding the particulate; an example of this method can be found in U.S. Pat. No. 5,964,291 issued to Bourne et al. Methods wherein the scale inhibitor is placed onto a porous medium, such as a particulate, are limited in that they can only carry a relatively small amount of scale inhibitor. Still another known method of placing scale inhibitor is to adsorb a scale inhibitor only the surface of a particulate. Similarly to the porous particulate method, this method is limited in that they can only carry a relatively small amount of scale inhibitor. Finally, methods are known in the art to place an insoluble or sparingly soluble salt of a scale inhibitor into a subterranean formation; however, these methods may not always have the desired mechanical properties.
Oftentimes, an area of a subterranean formation where scale inhibition is desired is also an area where it is desirable to place particulates. Particulates are often used in subterranean treatment operations. For example, hydrocarbon-producing wells are often stimulated by hydraulic fracturing treatments wherein, a carrier fluid, known as a fracturing fluid, is pumped into a well bore penetrating a subterranean formation at a pressure sufficient to create or enhance one or more cracks, or “fractures,” in the subterranean formation. Often, these fracturing treatments include particulates, often referred to as “proppant,” that are suspended in the fracturing fluid and deposited in the fractures. The proppant particulates may function to, among other things, prevent one or more of the fractures from fully closing upon the release of hydraulic pressure, forming conductive channels through which fluids may flow to the well bore. Hydraulic fracturing operations are well known in the art.
Another subterranean operation that uses particulates is a gravel packing operation. Gravel-packing operations generally comprise placing a screen in the well bore and packing the surrounding annulus between the screen and the well bore with gravel of a specific size designed to prevent the passage of formation sand. The screen may comprise a filter assembly used to retain the gravel placed during the gravel-pack operation. A wide range of sizes and screen configurations are available to suit the characteristics of the gravel particulates used. Similarly, a wide range of sizes of gravel particulates are available to suit the characteristics of the unconsolidated particulates in the subterranean formation. To install the gravel pack, the gravel may be carried to the formation in the form of a slurry by mixing the gravel particulates with the appropriate treatment fluids. The resulting structure presents a barrier to migrating sand from the formation while still permitting fluid flow. In addition to the traditional screened gravel packing operation, screenless gravel packing operations are well known in the art. By way of example, some screenless gravel packing methods are described in U.S. Pat. No. 6,745,159, the entire disclosure of which is hereby incorporated by reference.
In some situations, hydraulic fracturing and gravel packing operations may be combined into a single treatment. Such treatments are often referred to as “frac pack” operations. In some cases, the treatments are generally completed with a gravel pack screen assembly in place with the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In this situation, the hydraulic fracturing treatment ends in a screen-out condition, creating an annular gravel pack between the screen and casing. In other cases, the fracturing treatment may be performed prior to installing the screen and placing a gravel pack. Frac packing operations are well known in the art.
In some situations, the solid scale inhibitor of the present invention may be placed in the annulus, in the well bore itself, or in the rat hole of a well.